A hybrid seesaw model and hierarchical neutrino flavor structures based on A 4 symmetry

We propose a hybrid seesaw model based on $A_{4}$ flavor symmetry, which generates a large hierarchical flavor structure. In our model, tree-level and one-loop seesaw mechanisms predict different flavor structures in the neutrino mass matrix and generate a notable hierarchy among them. We find that such a hierarchical structure gives a large effective neutrino mass that can be accessible by next-generation neutrinoless double beta decay experiments. Majorana phases can also be predictable. The $A_{4}$ flavor symmetry in the model is spontaneously broken to the $Z_{2}$ symmetry, leading to a dark matter candidate that is assumed to be a neutral scalar field. The favored mass region of the dark matter is obtained by numerical computations of the relic abundance and the cross section of the nucleon. We also investigate the predictions of several hierarchical flavor structures based on $A_{4}$ symmetry for the effective neutrino mass and the Majorana phases, and find characteristic features depending on the hierarchical structures.